The present invention relates generally to a rotor supported in a housing, and, more specifically, to measuring axial or linear position thereof.
In U.S. patent application Ser. No. 08/686,618, filed Jul. 19, 1996, now U.S. Pat. Ser. No. 5,924,975 a linear pump is described for pumping blood as a left ventricular assist device (LVAD) for a human patient. The pump includes a tubular piston inside a tubular housing with a pair of check valves cooperating therewith for pumping blood therethrough as the piston reciprocates in the housing.
Reciprocation is effected by incorporating a linear motor having axially adjoining stator drive coils disposed in the housing which cooperate with a pair of axially spaced apart permanent magnet rings disposed in the piston. A linear position sensor is located in the housing for detecting axial position of the piston for use in commutating or controlling piston movement in pumping the blood.
An exemplary position sensor is in the form of a magneto resistor which cooperates with grooves machined in the outer diameter of the piston. In the presence of a magnetic field, the resistance of the magneto resistor increases dramatically and may be used to detect the grooves in the piston to thereby determine piston axial position in the housing for commutating the drive coils.
In the preferred embodiment of the linear pump, the piston also rotates in the housing to develop a journal bearing therebetween in which the blood fluid is pressurized in the journal bearing gap or annulus to suspend the piston and prevent contact with the housing bore.
In a further development of the pump, a rotary position encoder is used to measure the rotation angle of the piston for controlling its rotation during operation. A non-contact rotary encoder uses a Hall effect magnetic field sensor to detect the rotary position of the piston. However, the magneto resistor linear position sensor and the Hall effect rotary encoder are incompatible with each other since they both depend on magnetic fields.
Accordingly, it is desired to provide a linear position sensor unaffected by magnetic fields.